A closely coupled (minimum distance of 0.14 ${\lambda }_0$) Multiple Input Multiple Output (MIMO) antenna with high isolation is proposed. The MIMO antenna comprises of two circular-shaped radiating elements, which are fed by two opposite directed microstrip lines. The ground plane consists of modified Z-shaped metallic plane. Two different techniques are explored for improvement of isolation of MIMO antenna. First, a double ring structured frequency selective surface (FSS) is positioned above the antenna. The combination of FSS & antenna structure works with high isolation ($S_{21}$or $S_{12}$) (peak isolation of 49 dB) across the entire operating range (3.8 GHz–5.3 GHz). Secondly,arectangular metallic plate of thickness 0.3 mm is placed vertically between the MIMO antenna, which covers 4 GHz–5.3 GHz bands with high isolation (peak isolation of 49.4 dB). The proposed design consists of the MIMO antenna with both FSS and metallic plate. The proposed structure provides an appreciably high isolation (peak isolation of 59.5 dB) and peak gain of 4 dBi with Envelope Correlation Co-efficient (ECC) lower than 0.004. The proposed antenna having dimension of 40 mm $\times$ 40 mm is tremendously suitable for high isolation wireless applications.

紧密耦合（最小距离为0.14 ${\lambda }_0$）提出了具有高隔离度的多输入多输出（MIMO）天线。MIMO天线包括两个圆形辐射元件，由两个相对的定向微带线馈电。接地平面由修改后的Z形金属平面组成。为了改善MIMO天线的隔离度，探索了两种不同的技术。首先，将双环结构的频率选择表面（FSS）放置在天线上方。FSS与天线结构的结合具有很高的隔离度（${\mathrm{小号}}_{\mathrm{21岁}}$或者 ${\mathrm{小号}}_{12}$）（49 dB的峰值隔离）在整个工作范围（3.8 GHz–5.3 GHz）中。其次，将厚度为0.3 mm的矩形金属板垂直放置在MIMO天线之间，该天线覆盖4 GHz–5.3 GHz频段，具有很高的隔离度（峰值隔离度为49.4 dB）。拟议的设计包括带FSS和金属板的MIMO天线。所提出的结构提供了相当高的隔离度（59.5 dB的峰值隔离度）和4 dBi的峰值增益，而包络相关系数（ECC）低于0.004。建议的天线尺寸为40 mm $\times$ 40 mm非常适合于高隔离度的无线应用。